1. Field of the Invention
The present invention relates to a cleaning blade to be used for an image forming apparatus utilizing an electrostatic transfer process such as an electrophotographic copying machine, a laser-beam printer, a facsimile machine and the like.
2. Related Background Art
The electrophotographic copying machine and the laser-beam printer allow toners to adhere to an electrostatic latent image formed on a photosensitive drum and transfer them onto a copying paper, thereby performing the copying. As one of the methods of eliminating the loners remained on the photosensitive drum after the copying, a cleaning blade method has been put to practical use.
Heretofore in the past, as a material to be used for this cleaning blade, a thermosetting polyurethane resin of a polyester system urethane elastomer, which is excellent in mechanical strength such as abrasion resistance and the like, and which has little or no creeping property (permanent deformation due to abutting stress) has been used.
In general, the cleaning blade is often installed in a counter direction to rotation of the photosensitive drum and its mechanism is such that a free length of the cleaning blade is shrunk by rotation of the photosensitive drum and, by its stress, the photosensitive drum is strongly rubbed so that the residual toners are scratched off.
In the case where such a mechanism is adopted, accompanied with the recent speeding up of the electrophotographic process, when the circumferential speed of the photosensitive drum increases, a force to shrink the free length of the cleaning blade is strengthened and a shrinkage width thereof also increases so that the rubbing becomes often unstable. In order to maintain this shrinkage width adequately, it is necessary to reduce a loss factor of the viscoelasticity of a cleaning blade member. That is, it is necessary to effectively use the energy which was released when the cleaning blade was shrunk at the time when the cleaning blade was about to restore its original length. In this way, it can be considered that the shrinkage width of the free length of the cleaning blade is adequately maintained for the high circumferential speed of the photosensitive drum and a bounding can be prevented. For this reason, it is necessary that the elastomer material to be used for the cleaning blade have a molecular structure to optimize a ratio of the dynamic elasticity and the dynamic viscosity.
From such a viewpoint, for example, it is described in Japanese Patent Application Laid-Open No. 11-153934 that the urethane group concentration in specific polyester system urethane elastomer and the trimethylolpropane ramification group concentration are limited to a certain range to optimize a ratio of the elasticity and the viscosity of the component, and consequently, a cleaning blade having no bounding nor noise can be produced even at high process speed.
In general, urethane elastomer has a polyester molecular chain crystallized under low temperatures and enhanced in its hardness. For this reason, arrangement and orientation of the polyester chain is randomized, so that crystallinity of the polyester molecular chain is lowered, and crystallized temperature is reduced to give enough rubber elasticity even under low temperature ranges.
On the other hand, in recent years, high image quality and a full coloration of the electrophotographic copying machines and laser-beam printers have been advanced and, accompanied therewith, minuteness and sphericity of the toners have been advancing.
When minute toners and spherical toners are cleaned by the cleaning blade, it is necessary to control deformation of the cleaning blade smaller than usual. Accordingly, it is necessary to make a stress at the low deformation time larger than usual. In order to restore the original form from the deformation, it is also necessary to lower the dynamic viscosity of the rubber since the viscosity of the rubber prevents the restoration.
From such a viewpoint, it is described in Japanese Patent Application Laid-Open No. 11-265134 that, by using polyesterpolyol constituting urethane rubber of at least not less than two types, the crystal structure of polyester is disintegrated and the dynamic viscosity is controlled to a low level, thereby producing the cleaning blade having a strong property to return to the original form.
In addition to the above-described situation, in recent laser-beam printers and copying machines, while high durability has been pursued, the necessity to form a high definition image has been also increased. For this reason, it is necessary to further make the toners minute and, at the same time, to make a thickness of the surface layer of the photosensitive drum thinner. Accordingly, it is necessary for the cleaning blade to be able to satisfactorily clean the minute toners and lowly control abrasion of the photosensitive drum and have high durability by itself.
As an example of not damaging the surface of the photosensitive drum, it is described in Japanese Patent Application Laid-Open No. 7-98558 that the cleaning blade having a relatively low hardness of 60xc2x0 to 75xc2x0 in JIS - A standard (equivalent to 62xc2x0 to 78xc2x0 in the international rubber hardness (IRHD)) is good for abrasion of the photosensitive drum. However, as for abrasion of the photosensitive drum, there is not yet enough information and further improvement is strongly desired.
In view of the above-described situation, it is the object of the present invention to control a chipped edge of the cleaning blade and lower abrasion of the photosensitive drum and obtain a good image with improved durability.
According to the present invention to achieve the above-described object in a cleaning blade mainly installed in an electrophotographic apparatus and mainly comprising a polyurethane resin to rub and eliminate residual toners, the cleaning blade is provided wherein at least a prepolymer to be obtained from butylene adipate polyester polyol, hexylene adipate polyester polyol and polyisocyanate and a hardening agent including a polyol of low molecular weight and potassium acetate are mixed and hardened, so that a mol ratio (xcex1 value) of the hydroxyl group to the isocyanate group becomes not less than 0.7 and not more than 0.9.
Further, in the cleaning blade mainly installed in the electrophotographic apparatus and mainly comprising a polyurethane resin to rub and eliminate residual toners, the cleaning blade is provided wherein a prepolymer to be obtained from at least either one of butylene adipate polyester polyol or hexylene adipate polyester polyol and polyisocyanate and the hardening agent including at least either one of butylene adipate polyester polyol or hexylene adipate polyester polyol and a polyol of low molecular weight and potassium acetate are mixed and hardened so that both of butylene adipate polyester polyol and hexylene adipate polyester polyol are included and a mol ratio (xcex1 value) of the hydroxyl group to the isocyanate group becomes not less than 0.7 and not more than 0.9.
The present invention disintegrates the crystallinity of the polyurethane resin obtained by using a plurality of polyester polyols, so that a desired viscoelasticity characteristic can be secured. By using jointly potassium acetate and a universal catalyst from among temperature sensitive catalysts, a trimeric reaction of polyisocyanate is promoted and this is taken into polyurethane resin. As a result, the polyurethane resin which is low in hardness, small in elongation and large in modulus in a low deformed region is obtained. Further, the prepolymer and the hardening agent are mixed with an equivalent ratio (xcex1 value) of the hydroxyl group to the isocyanate group within a range of 0.7 to 0.9 so that a polyurethane resin which is low in hardness, small in elongation and large in modulus in a low deformed region is obtained.
As a result, the cleaning blade which is low in hardness, high in modulus and small in elongation is obtained so that deformation and turnover of the top end of the cleaning blade can be lowly controlled and abrasion of the photosensitive drum can be lowly controlled, thereby controlling the chipping off of the top end of the blade.